Update to pack size on add-batch.html (#442)

Changed answer options from Single vial/10 vials to Single item/10
items.

This is because from Sept 2025 we are also asking for pack size for flu
(if it's a CP or PCN and products comes in different pack sizes). And
not all products come in vials.

### Before

<img width="728" height="594" alt="Screenshot 2025-08-28 at 13 20 25"
src="https://github.com/user-attachments/assets/426e6c65-cd0f-4f0a-bd81-de3e37ccdaf9"
/>

### After
<img width="745" height="589" alt="Screenshot 2025-08-28 at 13 20 04"
src="https://github.com/user-attachments/assets/eaa80554-27a0-4ecd-bae8-fefcdc9628d3"
/>

Author: Anna-Sutton

app/views/vaccines/add-batch.html

@@ -59,14 +59,14 @@ <h1 class="nhsuk-heading-l">Add batch</h1>
             },
             "items": [
               {
-                "value": "Single vial",
-                "text": "Single vial",
-                checked: (data.packType == "Single vial")
+                "value": "Single item",
+                "text": "Single item",
+                checked: (data.packType == "Single item")
               },
               {
-                "value": "10 vials",
-                "text": "10 vials",
-                checked: (data.packType == "10 vials")
+                "value": "10 items",
+                "text": "10 items",
+                checked: (data.packType == "10 items")
               }
             ]
           }) }}